1. Field of the Invention
Aspects of the present invention generally relate to a sheet feeding device and an image forming apparatus, and specifically to a device that separates and feeds sheets by blowing an air to the sheets.
2. Description of the Related Art
In the related art, an image forming apparatus, such as a printer or a copying machine, includes a sheet feeding device that feeds sheets one by one loaded in a tray that supports a plurality of sheets. As the sheet feeding device, an air feeding method is known that blows air to an edge of a sheet bundle supported in the tray to float a plurality of sheets while loosening the sheets, and draws the sheet onto a suction conveyance belt disposed at the upper side to feed the sheet one by one. This technology has been discussed in U.S. Pat. No. 7,575,231.
In the meantime, a conveying method is discussed that separately conveys sheets one by one at a predetermined interval by a sheet feeding unit, while partially overlaying the sheets being conveyed. In this conveying method, the sheets are continuously conveyed by a conveyance belt and fallen at the downstream side of the conveyance belt. A delivery belt and a pressing roller are provided in an apparatus employing this conveyance method. The delivery belt receives and further conveys the fallen sheets. The pressing roller presses a leading edge of the sheet which is inclined between the conveyance belt and delivery belt.
In the conveying method that uses these belts, the delivery belt is driven at a sufficiently slower speed than the conveyance belt. Therefore, a sheet is conveyed in a state where a trailing edge of a preceding sheet is hanging on the conveyance while a leading edge thereof is hanging on the delivery belt such that a leading edge of a subsequent sheet is overlaid with the trailing edge of the preceding sheet. This technology is discussed in Japanese Patent Application Laid-Open No. 11-217148.
As described above, since the sheets are conveyed to overlay with each other in a stable state owing to the pressing roller that presses the leading edge of the sheet, it is possible to increase a conveyance amount of sheets in a unit time without increasing a driving speed of the roller or the belt in a conveyance path. Further, by suppressing the driving speed to be low, it is further possible to satisfy a certain performance even when a small size and low speed actuator is used as a driving source. Therefore, it is possible to suppress the increase in the cost, an operation noise, and power consumption to be low even when the actuator is driven at a high speed.
However, when the feeding unit of the air feeding method in the related art and a conveyance unit that conveys sheets to overlay with each other are combined, after separately delivering the sheets one by one from the feeding unit, the conveyance unit conveys sheets in an overlaid state to improve productivity. In this case, in order to improve the productivity by increasing a conveyance amount of the sheets, even though the conveyance amount of sheets is increased by the overlay conveyance in the conveyance unit, a feeding speed in the feeding unit that delivers the sheet to the conveyance unit goes up in proportion to a quantity of production. Therefore this method may not contribute to the lowering of the speed.
If a mechanism which conveys the sheets by the air feeding method while overlaying with each other is provided in the conveyance path, a size of the entire sheet feeding device becomes larger. Further, in order to perform overlay conveyance, two conveying speeds need to be provided, which may make apparatus control complicated.